1. Field of the Invention
The present invention relates to a method for obtaining a composite image using direct digital radiographic imaging. More specifically the invention is related to direct digital radiography using multiple wireless digital sensors.
2. Description of the Related Art
In traditional analog radiography, used e.g. in medical applications, imaging is performed by means of a light sensitive photographic film in combination with a phosphor layer which converts the incident X-rays to visible light. The light emitted by the phosphor is captured by the film which is developed to obtain an image on the film. Typically, different sizes of assemblies which may be conceived as cassettes or as film packages are used in daily practice ranging from a few squared cm (for e.g. dental applications) up to e.g. 35 cm*43 cm for a relatively large field of view used in chest X-rays. The drawback of film based systems is that they require that the photographic film has to be chemically processed, leading to chemical waste products and loss of time.
More recently, digital X-ray systems, now known as computed radiography (CR) systems use a stored energy releasing phosphor sheet which is exposed to a radiation image during X-ray exposure. The stimulable phosphor stores the radiation image at exposure where after the stored energy image is read out using stimulating radiation scanning the phosphor plate, releasing the image-wise stored energy as light. The light is detected and an electronic image is generated by the light sensor and processing electronics, where after it is digitized.
Digital radiography (DR) is a form of X-ray imaging, where digital flat panel detectors, also called DR detectors, are used instead of traditional photographic film or cassette based CR systems. Advantages include time efficiency through bypassing chemical processing (compared to traditional film based systems) and through immediate read-out of the image data from the sensor (compared to cassette based CR systems where the read-out of the detector is done by means of a dedicated digitizer system).
The advantages of the DR detector and DR detector assembly design include compact size and immediate access to digital images. The performance of DR systems greatly exceeds the performance of CR systems, which have conversion efficiencies of 20-35%, and of screen-film systems for chest radiography, which have nominal conversion efficiencies of 25%. Also the ability to digitally enhance images are important advantages of digital systems. DR detectors or flat panel detectors exist in 2 different categories; indirect and direct flat panel sensors. A DR detector is the active electrical component capable of capturing the digital image, and is the most important subcomponent of a DR detector assembly or cassette. The DR detector assembly comprises also the cassette housing, a battery, the read-out electronics, a memory, and the modules which support the wireless or wired data communication with the modality workstation.
Direct flat panel detectors convert X-ray photons directly into an electric charge. The outer layer of this type of detector in this design is typically a high-voltage bias electrode. X-ray photons create electron-hole pairs in a-Se (amorphous Selenium), and the transit of these electrons and holes depends on the potential of the bias voltage charge. As the holes are replaced with electrons, the resultant charge pattern in the selenium layer is read out by a TFT array, active matrix array, electrometer probes or microplasma line addressing.
Indirect flat panel detectors combine an a-Si (amorphous Silicon) detector with a scintillator in the detectors outer layer. The scintillator typically consists of caesium iodide (CsI) or gadolinium oxysulfide (Gd2O2S) which transforms the incident X-rays into visible light. This visible light is channeled through the a-Si layer which converts it to a digital output signal by means of a TFT (thin film transistor) array.
A DR detector is generally packaged in a suitable format for the application in an assembly (a “DR detector assembly” or “image sensor”). This DR detector assembly comprises the DR detector itself and at least the electronics to read-out the digital image. The DR detector assembly is designed to meet the form and fit requirements of the intended application.
Commercial versions of the aforementioned DR detectors are nowadays available in different dimensions and pixel resolutions, depending on the targeted clinical imaging application (such as dental, general radiology, mammography . . . ). However the maximum size of the DR detector is mainly limited by the maximum size that commercial production processes can produce at an acceptable cost. The reality of today is that commercially available DR detectors do not exceed the maximum traditional X-ray sensor sizes as being used throughout the different technology transitions described here above. One of the largest field of view sizes used in X-ray chest imaging, a radiology application which typically requires the largest possible field of view, is currently 17×17 inch.
Certain clinical applications such as “full leg” or “full spine” radiography however require larger fields of view in order to cover the entire body part under examination, a problem which traditionally is overcome by combining and arranging different cassettes or DR detector assemblies in such way that a contiguous, larger field of view can be obtained. This has been described in detail in EP0919858.
Since DR detectors have the advantage that they can be read out directly at the position of acquisition, the use of DR detectors for this application have led to a number of new approaches on how to extend the field of view of a DR based system.
One solution to perform large field of view acquisitions, is for instance to use a single movable DR detector assembly which is moved in between the acquisitions of the subimages in order to reposition the DR detector assembly for the next subimage acquisition. The DR detector assembly moves from one to the next sub image position to complete the entire desired field of view range. This technique has been described in U.S. Pat. No. 7,265,355. The technique has the advantage that only one DR detector assembly needs to be used, but has the disadvantage that the contiguous image has to be acquired in a series of sub-exposures, which induces the risk that the patient moves between the acquisitions of the different subimages, since the time to acquire these images takes several seconds, resulting in an uncontiguous patient image.
A better solution to obtain large field of view images is to use multiple partially overlapping DR detector assemblies simultaneously during a single exposure. The use of different DR detector assemblies simultaneously has the advantage that the large field of view acquisition is shot in one single exposure, which ensures that there is no risk for patient movement during the single acquisition.
This overlapping technique has been described in combination with DR detector assemblies in the art in EP1467226, and comes along with a number of intrinsic challenges which need to be overcome in order to obtain an acceptable image result. The said referenced publication EP1467226 describes how the images from the different DR detector assemblies can be acquired and digitally processed after acquisition in order to end up with a single digital image whereby the geometries of the subimages are adjusted, and the overlapping areas showing the edges of the DR detector assemblies are removed from the resulting image.
A particular technical characteristic of any DR detector is that (as opposed to a CR sensor or conventional screen film techniques) an active signal needs to be sent to the DR detector (called “trigger”) to signal it that the exposure will start. The trigger signal puts the DR detector in a state that it starts accumulating charges generated by the irradiation source, to build up the radiographic image. Also, the duration information of the exposure is sent to the DR detector to allow it to stop the image acquisition after a predetermined period of time. The reason to make sure that the acquisition time is limited to the exposure time only, is that in a DR detector, even in a state in which X-rays are not being irradiated, charges are generated due to dark current and alike, and these charges are accumulated in DR detector portions that detect the X-rays. Therefore, it is important to limit the acquisition time to the duration of the exposure. In most DR detectors, this acquisition state is preceded by an erase phase during which the unwanted accumulated charges causing background noise in the image are erased from the sensor, before it can be put in the acquisition phase.
Consequently, it is important that the DR detector is switched to its acquisition state as soon as the irradiation starts, and switches off when irradiation ends. In case the DR detector would be switched on too late, only a fraction of the irradiated dose would contribute to the radiographic image leading to a suboptimal image quality of the acquired image (or even worse, the acquisition would not detect any irradiation, and no image would be generated).
In a typical DR system, the DR detector is triggered to start the acquisition by means of an electrical signal running from the X-ray generator assembly to the DR detector. The duration of the acquisition is similarly communicated to the DR detector from the generator or X-ray system console.
Wireless DR detectors have become commercially available since 2009. Wireless DR detectors are DR detectors assemblies which can operate without being physically connected by a wire to a workstation. This type of DR detector assemblies are not integrated into a positioning device or a table, which means that they can be handled from a usability point-of-view in a similar way as CR detectors. With wireless DR detectors it is mandatory to use a wireless LAN for communications between the wireless DR detector and the workstation console. This way each performed radiograph is transferred at almost real time from the wireless DR detector to the workstation. The wireless DR detector assembly includes a built-in battery to supply power and this allows the sensor's necessary autonomy to obtain several radiographs and to transfer the obtained radiographs to the acquisition workstation for further viewing.
In wireless DR detectors, the trigger mechanism to start the acquisition needs to be generated within the wireless DR detector itself. This mechanism should pick up the early signs of an irradiation started at the X-ray source and hitting the wireless DR detector, and should trigger the wireless DR detector as soon as possible to start the acquisition state of the wireless DR detector. The many implementations described in the art mostly rely on mechanisms whereby a radiosensitive element in the wireless DR detector assembly detects incoming radiation, and will self-trigger the imaging sensor upon exceeding a measured radiation threshold value. U.S. Pat. Nos. 7,436,444 and 5,887,049 describe an implementation of such a self-triggering mechanism of a wireless DR detector whereby a dedicated radiosensitive element measures the incident exposure irradiation and will cause a driving circuit to generate the trigger as soon as a threshold value is exceeded.
US2014/0086391 describes a similar implementation of a self-trigger mechanism for a single wireless DR detector, but in this case multiple areas of the detector area of the DR detector are used to detect the incident irradiation. Different areas of the DR detector are used in order to have the area where the irradiation threshold is exceeded first trigger the entire sensor to start the image acquisition. This method allows for a higher probability to detect the start of the X-ray irradiation as soon as possible, and thus to self-trigger the wireless DR detector in time.
U.S. Pat. No. 6,972,411 describes another implementation of a self-trigger mechanism for a single detector in an intraoral application whereby the trigger signal is taken from a measurement of the total amount of current drawn from the entire detector. The entire area of the DR detector is used to monitor the start of the X-ray exposure.
US20140103220A1 discloses a triggering method applied within a DR detector assembly comprising 2 DR detectors, one which is intended for continuous fluoroscopic imaging, and the other to capture a higher resolution still image of the same patient area of interest, but at a higher resolution. The trigger method interrupts the fluoroscopic image capturing
The aforementioned necessity to accurately activate the acquisition time window for the DR detector needs also to be considered when performing large field of view radiography using multiple wireless DR detectors in a single exposure. In this case, it is important that all wireless DR detectors used in the large field of view exposure are put in their active modes as soon as the X-ray irradiation starts. For that to happen, all involved wireless DR detectors need to be identified correctly and need to be signaled individually to start the acquisitions of the different participating wireless DR detectors.
The problem which arises under the circumstances described above is that when using multiple independent wireless DR detectors, these wireless DR detectors upon exposure will be self-triggered independently from each other, which may cause that some of the wireless DR detectors may self-trigger (too) late to capture the maximum of the exposed dose or—in the worst case—may not self-trigger at all. Each individual wireless DR detector will self-trigger to start its acquisition only when a predefined (dose) threshold value will have been exceeded in one or more radiosensitive areas in the DR detector assembly, depending on the self-trigger mechanism in place. The time delay which the self-triggering mechanism suffers after the exact moment of the exposure start for each individual wireless DR detector will mainly depend on the position of the radiosensitive area of the self-triggering mechanism for each wireless DR detector in the radiation field.
The position of the radiosensitive area in the irradiation beam will determine the dose rate which is captured by the radiosensitive areas of the self-triggering mechanism. The relative dose rate of a certain position compared to another position in the irradiation field is determined by many factors, such as: the characteristics of the X-ray source (for instance, the “heel”-effect of a typical rotating anode tube causes a dose gradient in the irradiation field), any object (the patient, parts of the sensor assembly, collimator) attenuating the X-ray beam, the relative distance from the X-ray source, the relative coverage of the DR detector surface by the X-ray beam, etc . . . .
These differences will cause the individual identical wireless DR detectors to be triggered at different moments in time, which may lead—as previously explained—to suboptimal image quality across the sub-images, or—even worse—a loss of image.